1. Field of the Invention
The present invention relates to a technique for driving a liquid crystal display device, and more particularly to a liquid crystal panel driving method and apparatus of a dot-inversion system that is capable of constantly maintaining a quantity of a voltage applied to a liquid crystal cell.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) displays a picture corresponding to a video signal using a pixel matrix arranged at each intersection between gate lines and data lines. As shown in FIG. 1, each pixel includes a liquid crystal pixel cell (labeled “LC” in FIG. 1) for controlling a transmitted light quantity in accordance with a video signal, a thin film transistor 2 or 4 for switching the video signal to be applied to the cell LC from a data line DL, and a gate line GL for applying a gate driving signal so that the video signal from the data line DL can be applied to the cell LC. Also, the LCD is provided with gate and data driving integrated circuits (IC's) (not shown) for applying driving signals to the gate line GL and the data line DL, respectively.
Such an LCD has typically used three driving methods such as a frame-inversion method, a line-inversion method, and a dot-inversion method, so as to drive the liquid crystal cells LC of the liquid crystal display panel. In the frame-inversion driving method, the polarity of a data signal applied to each liquid crystal cell is inverted when a frame is changed. In the line-inversion driving method, the polarity of a data signal applied to each liquid crystal cell is inverted depending on the line in the LCD panel, that is, the polarity is inverted with respect to alternating gate lines. In the dot-inversion system, data signals having an opposite polarity are applied to adjacent liquid crystal cells and the polarity of a data signal applied to each liquid crystal cell is inverted every frame. Of the three LCD panel driving methods, the dot-inversion system allows a data signal having a polarity contrary to data signals applied to the adjacent liquid crystal cells in the vertical and horizontal directions to be applied to a certain liquid crystal cell, thereby providing a picture having a better quality than the frame- and line-inversion systems. In light of this advantage, recently LCD panels have mainly used the dot-inversion driving method or system. Dot-inversion systems are classified into 1-dot inversion systems and 2-dot inversion systems.
The 1-dot inversion system will be described in detail with reference to a waveform diagram of FIG. 2. First, a polarity pulse and a data output enable signal are each input to a data driving IC (not shown). In the 1-dot inversion system, the data output enable signal inputted to the data driving IC has twice the frequency of the polarity pulse. The data driving IC receiving the polarity pulse and the data output enable signal applies a video signal synchronized with the falling edge (or rising edge) of the data output enable signal to the data line DL. At this time, the video signal applied from the data driving IC to the data line DL alternately has a positive (+) polarity and then a negative (−) polarity alternately as shown in FIG. 2. Further, a gate output enable signal having the same frequency as the data output enable signal is applied to a gate driving IC. The gate driving IC generates a gate driving pulse by utilizing the gate output enable signal applied thereto and sequentially applies the generated gate driving pulse to the gate lines GL. In such a 1-dot inversion system, both the liquid crystal cells LC positioned adjacently having the gate line GL therebetween, and the liquid crystal cells LC positioned adjacently having the data line DL therebetween, are Supplied signals having an opposite polarity to thereby display a picture.
However, such a 1-dot inversion system has a large power consumption because all of the adjacent liquid crystal cells have a different polarity. In order to mitigate such a disadvantage, a 2-dot inversion system has been used.
The 2-dot inversion system will be described in detail with reference to a waveform diagram as shown in FIG. 4. First, a polarity pulse and a data output enable signal are input to the data driving IC. In the 2-dot inversion system, the data output enable signal input to the data driving IC has four times the frequency of the polarity pulse. The data driving IC receiving the polarity pulse and the data output enable signal generates a video signal synchronized with the falling edge (or rising edge) of the data output enable signal and applies the generated video signal to the data line DL. At this time, since the data output enable signal has four times the frequency of the polarity pulse, video signals are successively applied twice when the polarity pulse has a positive (+) polarity while video signals are then successively applied twice when the polarity pulse has a negative (−) polarity.
Further, a gate output enable signal having the same frequency as the data output enable signal is applied to the gate driving IC. The gate driving IC generates a gate driving pulse by utilizing the gate output enable signal applied thereto and sequentially applies the generated gate driving pulse to the gate lines GL. In such a 2-dot inversion system, as shown in FIG. 5, positive (+), positive (+), negative (−) and negative (−) polarities are alternately repeated in the vertical direction, while positive (+) and negative (−) polarities are alternately repeated in the horizontal direction. Accordingly, the 2-dot inversion system can reduce power consumption in comparison with the 1-dot inversion system in which an opposite polarity is applied to all of the liquid crystal cells LC.
In such a conventional 2-dot inversion system, however, a voltage value applied to a terminal “A” shown in FIG. 1 is different from a voltage value applied to a terminal “B” in FIG. 1. This will be described in detail, assuming that a positive (+) video signal should be currently applied to the data line DL while a voltage of 0V or less should have been previously applied to the data line DL. First, a gate signal is applied to the (n−1)th gate line GL, and a positive (+) video signal synchronized with the gate signal is applied to the data line DL. At this time, since a voltage of 0V or less has been applied to the data line DL prior to an input of the positive (+) video signal to the data line DL, a desired voltage rise time is required when the positive (+) video signal is applied to the terminal A. After the video signal is applied to the terminal A, a gate signal is applied to the nth gate line GL, and a positive (+) video signal synchronized with the gate signal is applied to the data line DL. In other words, a load on the data line when a video signal is applied to the terminal A is different from a load on the data line when a video signal is applied to the terminal B. Thus, as shown in FIG. 4, a voltage difference 8 is generated between a voltage applied to the terminal A and a voltage applied to the terminal B. Ultimately, even when the same video data is supplied, the same voltage is not applied to the liquid crystal cells LC positioned adjacently to each other to receive a video signal having the same polarity. This results in the LCD producing a cross line dimness, etc.